Recently, with the development of information and communication technologies and the digitalization of image information, an increasing number of electrical devices, such as a digital camera, a camcorder, a mobile phone, a personal communication system (PCS), a game machine, a security camera, and a medical micro-camera, are now equipped with image sensors. In general, an image sensor includes, among other things, a pixel region including pixels having photosensing elements such as photodiodes and a peripheral circuit region coupled to the pixel region. A unit pixel includes a photosensing element such as a photodiode that converts received light at the unit pixel into charges and a charge transfer transistor. The charge transfer transistor may be disposed between the photodiode and a floating diffusion region to transmit charges generated by the photodiode to the floating diffusion region.